The long term goal of this project is to determine the mechanism of gene regulation by nuclear receptors. We aim to focus mainly on Thyroid and Rednoid X Receptors as they are known to heterodimerize and form stable complexes with DNA response elements. DNA bound nuclear receptors associate with protein complexes that may either repress or activate the transcription through mechanisms that are not completely understood. Our aim is to isolate and characterize the novel interacting cofactors and to understand its functional and physiological relevance. To achieve this goal we have taken a biochemical approach to isolate and purify receptor interacting proteins on affinity columns. Large scale purification has allowed us to characterize three major proteins interacting with DNA binding domain of these receptors. The 65 kDa protein is a RNA binding protein TLS, that was originally identified as part of a fusion protein associated with human myxoid liposarcoma. The 105 kDa protein is a splicing factor known as PSF (PTB associated splicing factor). This protein is highly homologous to the third receptor interacting protein with a molecular weight of 50 kDa and is identified as NonO, also an RNA binding protein. PSF and NonO are components of a heterodimer and both genes have been reported in translocation involving a helix loop helix transcription factor gene, TFE3. In each case the fusion of almost the entire reading frame of PSF/NonO to TFE3 DBD results in renal cell carcinoma. Thus, the three RNA binding proteins that interact with nuclear receptors also turn into an oncogene when fused to a transcription factor and brought to an active transcription site. Our major goal will be to study the role of these protein factors in modulating the function of TR and RXR. These studies will also shed light on the possible mechanism by which PSF, NonO and TLS turn into oncogenes when fused to a transcription factor. The association of splicing factors with nuclear receptors that also participate in causing sarcomas when fused to a transcription factor is a novel and very interesting observation. Splicing factors might be playing a dual role through their functionally distinct domains and this study will provide evidences that transcription, RNA processing and oncogenecity might be intimately connected nuclear processes. We will also specify the region on the DNA binding domain of these receptors where these three proteins interact.